Flat Roof Insulation How To

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flat roof insulationLike most Southern Californians, we use our garage for just about everything EXCEPT storing a car. It’s a luxury not all parts of the country have, but it affords Californians with some square footage that might normally be a basement in other parts of the country. In our case the garage is a HomeFixated proving ground, workshop and all-around man-cave (you do need to ignore the washer and dryer, which completely mess with the man-cave feng shui). Our garage has a flat and dark-colored roof, and in the summer it turns into an oven that will literally broil you if dare set foot inside it mid-day. When we recently needed to reroof the garage, I decided to capitalize on the opportunity and install some rigid foam board insulation as well.

While rigid foam insulation board isn’t the most impressive form of insulation on the market, I didn’t have a lot of options to work with given our garage design. I could have used spray foam or even batts of insulation from the inside, but our open ceiling and exposed joists are filled with banks of fluorescent lights and various items stored in the cavities between the joists. That left us with insulating from the top, and given our flat roof and torch-down roofing material, rigid foam insulation seemed the most practical. While the rigid insulation added to the cost of our roofing contract, our roofer (Sequoia Roofing in San Diego) didn’t rake us over the coals. The insulation project was yet another “while we’re at it” event that regularly keeps us dumping more money than expected into just about every house project that comes up. Ah, sweet homeownership! Before we dive into the details, keep in mind best practices and building code vary widely, and what worked in our case may not be ideal or up to code in your neck of the woods.

Before the roofing and insulating project could get underway, our old roof needed to be demo’d. If you are ever having a flat roof redone like ours, make sure you cover everything you care about in the garage. Definitely remove your car entirely if you actually park yours in the garage. If you don’t cover things up, you’ll find all your possessions covered with a thick layer of soot and grime that will take you months to clean. I thought about tacking plastic sheeting directly to the ceiling joists, but our roofer said when people do that, the weight of the dirt inevitably pulls all the sheeting down along with the dirt. Luckily our roofer was conscientious and laid plastic sheeting over everything in the garage. With that done, the old layer of torch-down roofing and underlayment was ripped up and disposed of.


Flat Roof Insulation Installation
replacing-flat-roof-perimeterThe rigid foam board insulation we used was 2″ thick and came in 4′ x 8′ sheets. Once the roof demo was complete and the flat roof was cleared of any remaining debris, some rotted 2x material around the perimeter of the roof was replaced. The 1x roofing boards (which also made up the “ceiling” of the garage) had nice gaps that you could see the ceiling joists through. The joists were marked around the perimeter to make finding them much easier later. The 4’x8′ sheets of rigid foam board insulation were then laid out in a staggered pattern across the entire surface of the flat roof. Some parts of the country necessitate a vapor barrier under the rigid insulation, however this wasn’t necessary in our case.

Adding the Roof Sheathing
Sheets of 4’x8′ OSB were then laid directly on top of the rigid foam insulation boards, leaving small gaps per the manufacturer’s recommendation to avoid buckling at the seams. Using the perimeter marks for the ceiling joists, chalk lines were snapped to indicate where to screw down the sheathing. Square shaped “washers” were then laid out on the chalk lines. These distributed the pressure of the screw-head to a larger area and also recessed the head so it wouldn’t protrude into the roofing layers to be added above. Long enough screws were used so that the screws would go through the sheathing, insulation, 1x boards and securely into the joists below.

Chalk lines snapped to indicate joist location below

Finishing the Roofing
base-layerOnce a nice smooth surface of sheathing was in place on the flat roof insulation, a layer of 28# fiberglass base sheet was nailed in place with large head roofing nails. A single-ply granulated modified bitumen roofing material was then torched-down as the final layer of roofing material.

The final layer of roofing being torched down

The end result is a man-cave that feels more like a temperate cave and less like a broiler. Although the R-value isn’t huge, the rigid foam insulation boards do a nice job of keeping the searing heat of the roof from making its way into the sanctified space of the man-cave. Between the flat roof insulation and our install of Reach radiant barrier insulation on our garage door, the shop is now downright comfortable, even in the warmer months. We expect the flat roof insulation project to help retain heat in the winter as well. Of course “winter” here in San Diego isn’t going to do much to tax the insulation.

Sequoia-Roofing-Angies-ListOur thanks to Sequoia Roofing, who did meticulous work on both our garage roof and on a portion of tile roof we needed replaced. It’s worth noting that I found Mike and Sequoia Roofing after careful research on Angie’s List. As you can see from the screenshot I took before hiring, some companies reputations really shine in a review system such as Angie’s List.

A typical tsunami-like deluge of rain thwarted by our newly insulated flat roof
Photo of author

About Marc Lyman

Marc grew up under a brave single mom who "encouraged" home improvement on the family home. Early toddler gifts included a tool set, and even a cordless Bosch drill when cordless drills first came out. In grade school (give or take a few years), Marc's mom said, "We need to cut down some trees. . . . here's a chainsaw." A father figure also involved Marc in many home improvement projects, including a summer of home remodeling in Palo Alto, CA. Toss in some Obsessive Compulsive personality traits researching everything home improvement related. The end result: a genetically pre-disposed, socially sculpted home improvement machine! For his complete profile, please visit our About page. Really, it's worth it.

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16 thoughts on “Flat Roof Insulation How To”

  1. Aside from aesthetic considerations w/ roof color, everyone should remember that if you can see the roof from your window, there’s a good chance you’ll end up dealing with sunlight reflecting off of it at some point (maybe unless you live between skyscrapers or in a rain forest 🙂 .

    I ended up putting some white “Duck Coat” reflective elastomeric coating on some small, mostly-flat roofs on my very old farmhouse’s 1st floor bays. .. both to keep the 1st floor a little cooler in the summer, and to get a little more life out of my roofs.

    I knew reflectivity might be an issue but I figured, at worst, I’d only get a fraction of the reflected sunlight hitting the windows just above those bay roofs.. and only for part of the day (one’s on east side, other’s on west).

    That might be the case but having spent a summer working from home in one of the rooms above one of those roofs.. I can say that my little white roof throws a LOT of heat up through the window above it.

    So sure,.. my first inclination after reading this posting was to suggest you just go out and get 5-gal bucket of white or reflective roof coating. It can make a big difference.
    But, you need to be mindful of other considerations.. like is ‘painting’ your garage roof white going to turn it into a a solar mirror that converts your upstairs rooms into solar furnaces?

    BTW.. it doesn’t help in my case that my house was built in 1882. My upstairs windows are big.. ~30″ wide by 60″ tall.. and they’re original.
    At least, the one on the east side is original.. I pulled the west-side one out and installed a sheet of double-wall lexan panel (Insulated Greenhouse panel) in place of the old sashes.. until I get around to making new replacement sashes.
    Even through that panel (edges sealed, foam gasket all around) is WAY better than the old window in terms of thermal insulation.. I could still instantly tell when the sun came out in the afternoon.
    The ultimate problem is.. I’m not just (mostly) dealing with heat radiating though. The 2-layer greenhouse wall and storm window work really well together. It’s that i’m dealing with reflected sunlight.. which passes through the storm and panel. Even with a light shade in the window.. that reflected sunlight just heats up the shade.. turning into a radiator.


    • Hi Steven! Thanks for taking the time to share your thoughts and experience. You bring up great points regarding reflectivity to nearby occupied rooms. Thanks again for sharing – we welcome your comments anytime!

  2. Why did you raise the insulation with wood slats instead of laying the insulation directly over substrate? Also, why insulation under decking and not on top? We are doing a low slope modified torch down and are not wanting to have to sandwich insulation in with decking on bottom and top but still want to Also, is this insulation flammable?

    • The wood slats were added to improve the slope of the existing roof, not because they were required for the insulation installation. Our old flat roof was a bit too flat, so the slats actually created a bit more slope where needed. The insulation is under the sheathing because torching down onto insulation is probably not a great idea, nor is walking on a torched-down roofing material that is directly on top of relatively soft “foam.” Not sure about the flammability of the insulation. . . my guess is it likely is flammable, but I would recommend you check the spec’s on whatever specific type of insulation you are considering for flammability data. Good luck with your project!

  3. Marc. Barry again. I forgot to ask how you attached the rigid foam board to the garage roof? Did you nail it down?

  4. Marc I also live in San Diego and am putting an addition on my house with a flat roof. The builder already has OSB sheathing down, and I belatedly have asked him to put either 2 or 4 inches of iso rigid foam board over the sheathing before he applies the torch down roof. I greatly appreciated your article because, I was never sure about applying the torch down roof directly to the iso foam board. When I saw that you went in and applied more OSB over the foam board it has given me the idea of how to do this. Thanks again.

  5. I am a Roof and Solar installer. My suggestion is to always use a smooth torch down single ply, then apply a Elastomeric roof coating for sun protection. Simple reason. when torching with smooth vs graulated you have a great advantage of working the material from top. when you trowel or otherwise work from the top of granular material it marks up or smudges. you can work-rework areas of with smooth, cant do that with granulated. The whole idea of a torch down is to make it a smooth seamless roof. every seam should be troweled smooth and seamless, working around pipe, vents, AC’s, or other needs to be worked to detail. granulated its not ideal for detail. The only place i use granulated is wide open roofing, no detail work, and where you get a consistent tar flow at seam. Elastomeric coating works great as top coating for smooth. if you need custom color just add color to white. i have put down 1000’s of rolls.

  6. Loved the way you showed how to insulate a flat roof, but I need a drip edge. Like 3″x3″. Home Depot and Lowes do not sell it. Any ideas where I could buy it? I’m in Covina CA. Thank-you

    • Try looking up sheet metal and/or gutter supply shops, or roofing supply stores. If they don’t have what you need, they should at least be able to point you in the right direction. Good luck!

    • Hey Ethan! Well, we used a radiant barrier on the garage door, but the re-roof was done prior to me getting hip to radiant barriers. I suppose I could still do a barrier on the open ceiling now (from the inside), but the insulation we added did the trick. . . . at this point it would be overkill. But for warm climates like ours, radiant barriers can definitely be useful.

    • Lighter would definitely make a difference and many similar roofs in our area are white, light grey or even silver. The color choice was an aesthetic one. The roof is very visible from one side of the house – a light color would show dirt conspicuously and wouldn’t tie in with the look of the house. We definitely considered it, but the insulation resolved the issue without having to change the color, and we got a more insulated garage in the process.


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